The present invention relates to a process for producing a metal-polyimide composite article and more particularly to a wiring board which exhibits a very small in delay time. The composite article of the present invention employs a polyimide which is low in dielectric constant and high in heat resistance as an insulating film and a conductor such as copper low in resistance in a wiring layer. The invention relates to materials which can be used for main high-density and high-speed wiring boards for future computers of from large size to middle or small size and flexible printed boards which are essential for miniaturization of electronic parts.
Hitherto, there have been used multi-layer circuit boards comprising an epoxy or maleimide resin-glass cloth laminate and copper foil and an alumina/tungsten multi-layered circuit boards as high density wiring board. However, in the former case, there are limitations in production methods and materials and fine wire width of less than 30 .mu.m cannot be obtained and in the latter case, tungsten of high heat resistance with high resistivity must be used because ceramics are high in dielectric constant and a very high temperature process is required for production. Thus, wiring boards of high performance have been demanded as future high-speed and high-density wiring boards to be used in place of the above conventional wiring boards. The first one is a copper/polyimide wiring board. This is produced by forming polyimide of low dielectric constant and copper of low resistivity on a silicon wafer or ceramic substrate by the same fine process as used for production of semiconductor wiring. By this method, it can be expected that wiring board of high performance is obtained.
However, there is the problem that when polyamic acid which is a polyimide precursor is heated and cured in contact with copper or silver at higher than 300.degree. C., a pyrolysis reaction occurs which cannot be considered to occur in view of the heat resistance of polyimide per se. For example, it has been known that when polyamic acid varnish is coated on a film of copper and heat cured, distinct discoloration is recognized at higher than 300.degree. C. and the film becomes mechanically very brittle. Similar phenomena are recognized to occur at production of flexible printed board or curing of electrically conductive silver paste. In case of metals, this problem is especially conspicuous for copper and silver and nearly no problem is seen in aluminum, titanium, nickel and chromium.
Up to now, in forming an imidized film in contact with a metal, the following methods have been employed, namely, a method of forming a metallic film such as of inert chromium or an inert film of SiO.sub.2 or Si.sub.3 N.sub.4 and coating thereon a polyamic acid precursor varnish and heating and curing the varnish and a method of heating and curing the varnish in a reducing atmosphere such as hydrogen when the varnish is allowed to directly contact with the metal. These methods suffer from severe problems such as much increase in the number of steps and increase in running cost.
Taking the hint from the fact that even if a film of such metal is formed on a polyimide film and heated, substantially no adverse effect is given to the polyimide film, the inventors coated a varnish of a polyamic acid on a metallic film and heated and cured the varnish and as a result have found that no deterioration of polyimide occurs. However, this method still has the problems that since solubility of polyimide is very poor, cresol type solvents which are harmful to human bodies must be used and only special solvents which are highly soluble in polyimide per se can be used.
As a result of investigation for solving these problems, it has been found that when polyamic acid of which carboxylic acid is esterified by an alcohol is used, no problem occurs. However, since this polyamic acid whose carboxylic acid is esterified is obtained by reacting an acid anhydride with an alcohol, then converting the product to an acid chloride with thionyl chloride and then further reacting the acid chloride with a diamine, there are many problems to be solved that preparation steps are much prolonged and ionic impurities are incorporated at preparation steps to cause troubles in use.